The present invention will be described with reference to a television receiver having a picture-in-picture feature. As mentioned in the parent case, conventional processing of interlaced television signals is based on the assumption that the even fields, (i.e. the fields furnishing the even lines) and the odd fields (furnishing the odd numbered lines) must be maintained as such throughout all processing. The parent case introduced the concept of top field and bottom field, a concept which preserves the spatial relationship between the two fields without requiring identification of odd and even fields.
As in the parent case, the specific embodiment of the invention to be described is that of a field generator incorporated in a picture-in-picture (PIP) television system.
In a PIP system, a television signal from a second channel or another source is sub-sampled to decrease its size and displayed within a larger television signal. Specifically, in the vertical direction, every third line is maintained, while the remaining two lines are dropped. Similarly, the picture is horizontally sub-sampled so as to decrease its width. This reduced picture is stored in memory in write cycles controlled by the PIP source synchronization signals.
In order to be displayed as part of the main TV signal, the PIP signal must be read from memory in synchronization with the main signal. The signal read from memory must then be combined with the main TV signal (herein, as an example, the main luminance signal) to yield a main luminance signal with inserted PIP.
In this system, the PIP source and main picture source are asynchronous and tend to drift relative to each other. Due to this drift, and because the read-out takes place faster than the recording in the memory, the display side will at some time want to read from memory the same line in the same field that is currently being recorded. For the remainder of the PIP, the information read from memory will be a thirtieth of a second older than the information immediately preceding it. The same will occur on subsequent PIPs. This causes visible distortion, particularly in action scenes and when a camera change occurs.